The blending of plasticizers in general with thermoplastics and the resulting plasticization of those thermoplastics are well known and many reviews have been written. A plasticizer is generally an organic compound incorporated into a high polymer, such as for example a thermoplastic, to desirably facilitate processing, increase its workability, flexibility, and/or distensibility of the polymer.
Within the last few years efforts have been made in the field of plasticizers to better understand factors which govern plasticizer/thermoplastic miscibility. Examples of a thermoplastic and plasticizer include polypropylene and low molecular weight polyolefins, respectively. Polypropylene is an inexpensive polyolefin engineering thermoplastic that is generally stiff and even brittle below room temperature especially for highly stereoregular polypropylene. Tackifiers are examples of low molecular weight polyolefins plasticizers. Examples of tackifiers include hydrocarbon resins derived from fractionated petroleum distillates, coal tar, turpentine fractions and from copolymerization of pure aromatic monomers. However, these tackifiers typically have high glass transition temperature (Tg) and high solubility parameter. As such, upon blending, these tackifiers tend to raise the Tg of the polypropylene. Increasing the Tg increases the stiffness of polypropylene.
Other plasticizers, which have low Tg, (below −20° C.) such as ethylene-propylene rubber, ethylene-butene copolymer (having a Mw greater than or equal to 20,000), are immiscible with the polypropylene. Plasticizers which are immiscible with polypropylene tend to collect on the surface of the manufactured article, hinder the manufacturing process of articles made therefrom and may cause the resulting product to have generally undesirable features.
Because polypropylene is an inexpensive thermoplastic, there exists a need to improve its workability and to overcome its inherent stiffness and brittleness which limit its commercial application. Therefore, a need exist to safely and economically improve the workability, flexibility, and/or distensibility of polypropylene.